Method optional

cranelift/reader/src/parser.rs:543–550

If the next token is a `want`, consume it, otherwise do nothing.

(&mut self, want: Token<'a>)

    // If the next token is a `want`, consume it, otherwise do nothing.
    fn optional(&mut self, want: Token<'a>) -> bool {
        if self.token() == Some(want) {
            self.consume();
            true
        } else {
            false
        }
    }

    // Match and consume a specific identifier string.
    // Used for pseudo-keywords like "stack_slot" that only appear in certain contexts.